1. Field of the Invention
The present invention relates to an objective lens for being used in an optical pickup device which can carry out the record and/or the reproduction of information for a plurality of optical information storing media in which the protect substrates have different thicknesses from each other; an optical pickup device; a recorder and a reproducer.
2. Description of Related Art
As a short wavelength red laser is practically used, a DVD (Digital Versatile Disk) which is a high density optical information storing medium (also referred to as “optical disk”) having the same size as a CD (Compact Disk) and a high capacity is commercialized.
In a recording/reproducing apparatus for CD, a numerical aperture NA on an optical disk side of the objective lens is about 0.45 when a semiconductor laser emitting a light having a wavelength of about 780 nm is used. On the other hand, in a recording/reproducing apparatus for DVD, a numerical aperture on an optical disk side of the objective lens is about 0.6 when a semiconductor laser emitting a light having a wavelength of about 650 nm is used.
The DVD has a track pitch of 0.74 μm and a shortest pit length of 0.4 μm. The DVD has a 4 or more times higher density than the CD having a track pitch of 1.6 μm and a shortest pit length of 0.83 μm. In the DVD, in order to suppress a comma aberration caused by inclining the optical disk to an optical axis, the thickness of the protect substrate is 0.6 mm and is a half of the thickness of the protect substrate of the CD (1.2 mm).
In addition to the CD and the DVD, optical disks having various standards in which the wavelengths of lights emitted from the light sources, the thicknesses of the protect substrates and the like are different from each other, for example, CD-R, CD-RW (write once type of compact disk), VD (video disk), MD (mini disk), MO (magneto optical disk) and the like, are commercialized and popularized. Further, the semiconductor laser emitting a light having a shorter wavelength has been developed. A short wavelength blue light laser having an oscillation wavelength of about 400 nm will be practically used. Because the wavelength becomes short, it is possible that the optical information storing medium has a higher capacity even though an objective lens having the same numerical aperture as the objective lens for DVD is used.
As described above, a plurality of optical information storing media having various recoding densities, thicknesses of the protect substrates for the recording layer and wavelengths of the lights emitted from lasers for record/reproduction have been developed. An optical pickup device which can carry out the record and/or the reproduction for two or more optical information storing media among these optical information storing media, has been demanded. Therefore, each type of optical pickup device which comprises a plurality of light sources corresponding to each using wavelength and condenses each laser light on a recording layer by using a single objective lens at a required numerical aperture, are proposed.
For example, in an optical pickup device which can carry out the reproduction and/or the record for a DVD and a CD, when the reproduction/record is carried out for a CD, a light ray passing through a region that a numerical aperture is larger than about 0.45 among an optical functional surface of an objective lens, is not used. Therefore, the optical pickup device having the objective lens that when a laser light having a wavelength for the reproduction/record for a CD passes through the above-described region, the light becomes a flare light and that in case of a laser light having a wavelength for the reproduction/record for a DVD, the aberration is hardly caused by using the laser light passing through the whole region of the optical functional surface of the objective lens, has been proposed.
In the above optical pickup device, for example, as shown in FIG. 1, the defocus/tracking is controlled by a main/sub sensor of a light receiving section. When the flare light enters a sensor, there is some possibility that a wrong operation relating to the reproduction/record for the optical information storing media is caused.